Capacitive proximity sensors are widely used in various applications. For example, such sensors are used in place of pushbutton switches in keyboards and control panels, as limit switches in various kinds of equipment, to detect the flow of dry and liquid materials in pipes and to detect motion of objects relative to each other. Various other kinds of control systems are also well known in which capacitive proximity effects are used. For example, an embodiment of a capacitive proximity sensor as a part of a position control system is disclosed in FIG. 8 of U.S. Pat. No. 4,893,071 to Gabriel L. Miller.
Systems using capacitive proximity sensors typically measure the capacitance, or change in capacitance, of a sensing electrode with respect to its environment as an object being sensed moves in relation to the electrode. Typically, the sensing electrode is connected to an alternating current or radio frequency (RF) source and the current to the electrode is monitored to detect such capacitance changes.
In the design of such a system, care must be taken to shield not only the sensing electrode but also the wiring to such electrode from stray capacitances to ground or to other conductors. Depending on the application for the sensor, such shielding may be simple or complex. Typically, the sensing electrode is protected by a shielding or guard electrode that is shaped to protect the sensing electrode. One example is a sensing electrode and a guard electrode formed on different layers of a multi-layer circuit board as shown in U.S. Pat. No. 4,766,368 to Harold A. Cox.
Because of the wide variety of applications for capacitance proximity sensors, it is desirable to be able to form sensing electrodes in various shapes and sizes. Because of the need to shield such electrodes, it is also desirable to form the shield together with the electrode. Further, it is desirable to have electrical means that are simple and reliable for driving such shielded electrodes and that can be used with various shapes, sizes and types of electrodes.